Method for selectively covering a micro machined surface

ABSTRACT

On a die that has etchings on a surface, firstly a sheet of negative photoresist is laid down which, by means of an exposure and subsequent development, is left only above the etchings; then, upon the negative photoresist, a positive photoresist is applied, which is subjected to exposure and development to produce functional geometries deposited in thin film; subsequently the positive photoresist is removed in a “lift-off” operation, and the negative photoresist is taken off in a plasma operation, thus revealing the etchings.

This application is a National Stage application of co-pending PCTapplication PCT/IT2003/000545 filed 11 Sep. 2003, which was published inthe English language under PCT Article 21(2) on 25 Mar. 2004, and claimsthe benefit of Italian application number TO2002A00793 filed 12 Sep.2002. The disclosures of each are expressly incorporated herein.

TECHNICAL FIELD

This invention relates to the mems 3D technology (mems: MicroElectro-Mechanical System), or the moems 3D technology (moems: MicroOptical Electro-Mechanical System), and more precisely related to amethod for uniformly laying a photoresist on a die having an irregularsurface, for example on account of its having etchings or recesses.

BACKGROUND ART

Depicted in FIG. 1, as a non-restrictive example of application of themoems technology, is an axonometric illustration of an optoelectronicdevice 10, integrated on a die 51 which, by way of non-restrictiveexample, may be of a semiconductor material, generally silicon. On theupper face 11 of the die 51, etchings 12, needed for instance for theseating of optical fibres, are made. Also on this upper face 11, thereare outer pads 52 to which optoelectronic components 53 have beensoldered, and inner pads 54 needed for connecting the optoelectronicdevice to an external circuit not shown in the figures.

Still in FIG. 1, the x, y and z axes which give the three-dimensionalreferences of the die 51 are defined.

The process for manufacturing the optoelectronic device will now bedescribed according to a technique known as “lift-off” to thoseacquainted with the sector art, by means of which geometries havingdifferent layers can be produced directly on non-reactive materials,such as for example titanium, platinum, gold, or with which solderingalloys (for example gold/tin 80/20) may be deposited selectively, ornon-metallic materials may be deposited selectively. Reference should bemade to the flow diagram of FIG. 2, for only the steps necessary for theunderstanding of this invention.

In a first step 70, a wafer 66 is made available on which the dice 51are made (FIG. 3).

In a step 71, illustrated with the aid of FIG. 4, on the dice 51 a layer61 is spread of “lift off resist”, for instance of the LOR® series byMicro-Chem, having for example a thickness of between 0.5 and 6 □m, asindicated in the cross-section detail of FIG. 4 which concerns an areawithout etchings. The lift off resist is usually applied in the fluidstate, by means for instance of a centrifuge in a process known as“spinner coating”.

In a step 72, on the lift off resist 61, a layer 60 of conventional typepositive photoresist is laid, having for example a thickness of between0.5 and 20 □m, as also indicated in FIG. 4. The positive photoresist isalso usually applied in the fluid state, by means for instance ofspinner coating.

Designated with number 14 is the upper face of the layer 60 ofphotoresist, substantially parallel to the face 11 and to the x, y axes.

The photoresist is defined as “positive” if, starting from an initiallyinsoluble state in its development solvent, it depolymerizes due to theeffect of radiation, for example ultraviolet, becoming soluble.

The technology described, which uses the layer 61 of lift off resist andthe layer 60 of conventional positive photoresist, is called “bilayer”.

In a step 74 (FIG. 5), exposure of the photoresist is performed toultraviolet (UV) radiation by means of a mask 13 provided with windows122. The photoresist, being positive, depolymerizes in the zones 26corresponding to the windows 122, and therefore struck by the radiationUV, whereas it remains insoluble in those zones that are kept in theshade by the opaque areas of the mask.

In a subsequent step 75 the layer of photoresist 60 is developedaccording to known techniques which, by means of a solvent, remove thephotoresist only in the zones 26 depolymerized by ultraviolet (UV)radiation through the windows 122: thus cavities 64 are made, bounded byedges 25 (FIG. 6).

The same solvent dissolves the underlying lift off resist 61 to agreater extent than the photoresist 60, thereby producing sub-etchings22, the depth of which depends on the development time.

A first alternative exists, illustrated in FIG. 7, according to whichonly the monolayer layer of photoresist 60 is produced. Afterdevelopment, the cavities 64′ in this case are bound off by walls 15.

On account of diffraction and reflection phenomena in the UV radiation,the depolymerization of the layer of photoresist 60 takes place,parallel to the plane x-y, to a greater extent in the vicinity of theface 11 of the die, and to a lesser extent in the vicinity of thesurface 14 of the photoresist: the walls 15 are not therefore parallelto the z axis, but instead have an undercut β, positive according to thesign convention indicated in FIG. 7. This technology is called“monolayer”, and is less expensive but also controllable with much lessprecision than bilayer.

There is also a second monolayer alternative, again illustrated with theaid of FIG. 7, according to which only the layer of photoresist 60 isproduced, which is then treated with a surface modifier, for instancetoluene, which renders the upper surface 14 more resistant to thesolvent: in this way, after development, in the same photoresist 60,walls 15 with sub-etching or with a more pronounced positive undercut βare produced. Costs and controllability of this second alternative liebetween those of the first monolayer alternative and those of thebilayer technology.

In a step 76 a vacuum deposit is made, for instance of a metal, on thesurface 14, in which the latter remains intact, and on the face 11 wherethis is uncovered through the effect of the development described instep 75. The deposit takes the form, for example, of a “sputtering” or“electron beam” process, both of which are known, the result of which isa subassembly 23, illustrated in FIG. 8, comprising a first depositedlayer (52, 54) adherent to the face 11, which assumes the geometry ofthe cavities 64 and effectively constitutes the outer pads 52 and theinner pads 54, and a second deposited layer 16 adherent to the surface14. No layer is deposited on the sub-etchings 22.

Even when the monolayer technology is adopted, no layer is deposited onthe walls 15, thanks to the positive undercut β and to the sub-etchings,if any.

This separation between the two deposited layers is essential forsubsequent operations, and is a first fundamental reason for choosingpositive photoresist to produce the layer 60, as there is in fact nopractical possibility of producing a bilayer with negative photoresist,whereas, if a monolayer with negative photoresist is chosen, theundercut β would be negative according to the sign convention adopted,and the walls of the cavity would be covered over by the deposit.

The deposited layers 52, 54 and 16 may be metallic, or made ofnon-metallic materials, such as for example oxides, nitrides, carbidesand the like.

If the deposited layers 52, 54 and 16 are made of metals, these may benon-reactive type, such as for instance titanium, gold or platinum, forthe outer pads 52 or the inner pads 54, or a gold/tin 80/20 alloy forthe soldering. These alloys are generally deposited, according to aknown technology, in alternate layers of the component metals: thevarious layers are produced in appropriate ratios between thethicknesses to give the alloy—generally eutectic—the right composition,and can have an overall thickness, for instance, up to 5 μm.

In a step 77, illustrated with the aid of FIG. 9, the layer 60 ofpositive photoresist and the layer 61 of lift off resist are removed bymeans of a process known as “lift off” to those acquainted with thesector art. The subassembly 23 is plunged into a solvent 26, forinstance acetone which, through the edges 25 and the sub-etchings 22,free of the deposited layer, penetrates through the layers 60 and 61,and dissolves them, as indicated by the arrows 21, eliminating themcompletely and freeing the second deposited layer 16, which is then castaside.

The operation is facilitated by a mechanical action such as, forinstance, an ultrasound wash, and can only be conducted on a positivephotoresist: this is a second fundamental reason why positivephotoresist is chosen for the layer 60. If, on the other hand, anegative photoresist were to be chosen, it would not be possible toperform the lift off operation with today's technologies.

Where the monolayer technology is selected, this step 77 is performed inthe same way, since the solvent can penetrate through the walls 15, alsofree of the deposited layer.

At the end of the step 77 the subassembly 23 is finished, as shown inFIG. 10, where a die 51, an outer pad 52 and an inner pad 54 may beseen.

This process, however, has a number of technical problems that will nowbe described.

When etchings 12 are made on a face of the die 51, as indicated in thesection view of FIG. 11, a few tenths or hundredths of a μm thick forinstance and needed typically in moems applications for seating opticalfibres, the layers 60 and 61 are distributed unevenly on account of theetchings 12.

This produces an insufficient definition of the figure to be removedduring the exposure and development, which makes the process practicallyimpossible to use.

In particular, the etching 12 may reach a depth D of a few hundredths ofa μm. Furthermore, if the die 51 is made of silicon, the etching 12 isoften obtained by way of a chemical reaction which advances according tothe crystallographic axes of the silicon, forming two walls 20 whichproduce an angle α=54.7° with respect to the x axis: the width W of theetching is therefore:W=2 D/tan α

If, for example, the etching concerns roughly one half of the thicknessof a wafer of 625 μm, it reaches a depth D of about 300 μm; in thiscase, W=425 μm, a width that makes the above-mentioned unevennessextremely serious.

Moreover, choice of the positive photoresist is dictated by the firstand second reasons already illustrated, and the layer of positivephotoresist is normally applied in the liquid state: this favoursunevenness of the deposition local to the etchings.

A second technical problem also exists: in some subassemblies, such asfor instance those made in the moems technology, etchings 12 must bemade on the same face as that containing other films. If the etching ismade after the films are deposited, it is necessary to protect the filmsduring the chemical reaction on the silicon, which is highly aggressiveas it uses KOH or TMAH for numerous hours at a temperature of roughly80° C., as is known to those acquainted with the sector art.

It is therefore advantageous to make the etchings in the silicon at thestart of the process, for instance through a known process with a maskof SiO2, and to deposit and define the films at a later time, but inthis way the first of the problems outlined crops up again.

DISCLOSURE OF THE INVENTION

The object of this invention is that of selectively depositing a layeron a die that has an irregular surface, in particular on account ofetchings, according to a predefined geometry.

Another object is that of selectively depositing the components of asoldering alloy in alternating layers.

A further object is to maintain the etchings clear and clean throughoutthe entire die manufacturing process.

Yet another object is that of depositing a layer of positive photoresiston a die that has an irregular surface, in particular on account ofetchings, in a layer of uniform thickness.

The above objects are achieved by means of a method for selectivelycovering a micro machined surface, characterized as defined in the mainclaims.

These and other objects, characteristics and advantages of the inventionshall become apparent from the following description of a preferredembodiment, provided as a non-restrictive example, and with reference tothe accompanying drawings in which:

FIG. 1 represents an axonometric view of an optoelectronic device;

FIG. 2 illustrates the flow of one part of the manufacturing process ofthe optoelectronic device of FIG. 1, according to the known art;

FIG. 3 represents a wafer of semiconductor material, containing dicethat have not as yet been separated;

FIG. 4 represents a section view of a die without etchings, bearing alayer of lift off resist and one of positive photoresist;

FIG. 5 represents the exposure of the layer of positive photoresist onthe die;

FIG. 6 represents the development of the same layer of photoresist;

FIG. 7 represents the development of the layer of photoresist in themonolayer technology;

FIG. 8 represents a deposited layer on the layer of photoresist and inthe cavities;

FIG. 9 represents the lift off of the layer of photoresist;

FIG. 10 represents the subassembly at the end of the process described;

FIG. 11 represents a section view of a die provided with an etching, andbearing a layer of lift off resist and a layer of positive photoresist,according to the known art;

FIG. 12 illustrates the flow of the manufacturing process of theoptoelectronic device with etchings, according to the invention.

FIG. 13 represents a wafer of semiconductor material, containing dicewith etchings;

FIG. 14 represents a section view of a die with an etching, and bearinga film of negative photoresist;

FIG. 15 represents the exposure of the film of negative photoresist;

FIG. 16 represents the development of the film of negative photoresist;

FIG. 17 represents the application of the layers of lift off resist andof positive photoresist;

FIG. 18 represents the exposure of the layer of positive photoresist;

FIG. 19 represents the development of the same layer of photoresist;

FIG. 20 represents the development of the layer of photoresist in themonolayer technology;

FIG. 21 represents a deposited layer on the layer of photoresist and inthe cavities;

FIG. 22 represents the lift off of the layer of photoresist;

FIG. 23 represents the subassembly at the end of the process described,according to the invention.

DESCRIPTION OF THE PREFERED EMBODIMENT

The manufacturing process of the optoelectronic device according to thisinvention is now described, with reference to the flow diagram of FIG.12, limited to those steps needed for understanding of the invention.

In a step 170 a wafer 66′ is made available, containing dice 55 (FIG.13) which have etchings 12, as illustrated in the enlarged portion ofthe same FIG. 13. In the description, it is assumed, as anon-restrictive example, that the die 55 is of silicon, though the samedie could also be made of other materials, such as for instance glass,ceramic or other insulating materials, or of GaAs or othersemiconducting materials, or of metals.

In a step 140, a film 30 of negative photoresist is spread on the die55, as seen in section view in the FIG. 14. Thickness of the film 30 is,for example, between 5 and 30 μm, and it is sufficiently rigid to coverthe etching 12 without settling on it. The etching 12 is in this wayprotected, and at the same time remains clean as it has no contact withthe photoresist. The upper surface of the film 30 is designated by meansof the numeral 35.

The photoresist is called “negative” if, starting from a state solublein one of its development solvent, it polymerizes through the effect ofradiation, for example ultraviolet, becoming insoluble.

In a step 141, (FIG. 15), exposure is performed of the film 30 ofnegative photoresist to ultraviolet (UV) radiation by means of a firstmask 31 fitted with a window 32. The window 32 extends above the etching12 and above a margin around this, a few tenths of a μm wide forinstance. The photoresist, being negative, polymerizes in a zone 27corresponding to the window 32, and therefore struck by the UVradiation, whereas it remains depolymerized in the zones held in theshade by the opaque portions of the mask 31.

In a later step 142, development is performed of the film 30 of negativephotoresist according to known techniques which, by means of a solvent,remove the film only in the zones that were depolymerized (FIG. 16): incorrespondence with the zone 27, a covering 33 remains, bounded byflanks 34, which covers the etching 12. On account of diffraction andreflection phenomena of the UV radiation, the polymerization of the film30 takes place, parallel to the x-y plane, to a greater extent in thevicinity of the face 11 of die, and to a lesser extent in the vicinityof the surface 35 of the film: the flanks 34 are not therefore parallelto the z axis, but have a taper γ, negative according to the signconvention indicated in FIG. 16.

In a step 171, similar to step 71 already described in relation to theknown art, on the dice 55 a layer 161 of lift off resist is applied, forinstance of the LOR® series by Micro-Chem, having for example athickness of between 0.5 and 6 □m, as indicated in the section in FIG.17. The lift off resist is usually applied in the fluid state, by meansfor example of the process known as spinner coating.

The taper γ of the covering 33 promotes a better application of the liftoff resist, and also a better flattening of it.

In a step 172, similar to the step 72 already described for the knownart, on the lift off resist 161 a layer 160 of conventional positivephotoresist is applied, having for example a thickness of between 0.5and 20 □m, as indicated in the same FIG. 17. The positive photoresist isalso usually applied in the fluid state, by means for example of spinnercoating.

Designated with the numeral 114 is the upper surface of the layer 160 ofphotoresist, substantially parallel to the face 11 and to the x-y plane.

With this method, thanks to the presence of the covering 33, the uppersurface 114 is more regular than the surface 14 in the known art, andtherefore permits cavities with controlled dimensions to be made, evenin the vicinity of the etchings.

In a step 174, similar to the step 74 already described for the knownart, the exposure of the positive photoresist to ultraviolet (UV)radiation is performed by means of the mask 13 provided with windows 122(FIG. 18). The photoresist, being positive, depolymerizes in the zones26 corresponding to the windows 122, and therefore struck by the UVradiation, whereas it remains insoluble in the zones that are kept inthe shade by the opaque areas of the mask.

In a subsequent step 175, similar to the step 75 already described forthe known art (FIG. 19), the layer of photoresist 160 is developedaccording to known techniques which, by means of a solvent, remove thephotoresist only in the zones 26 depolymerized by the ultraviolet (UV)radiation through the windows 122, so that in this way the cavities 64bound by the edges 25 are made.

The same solvent attacks the underlying lift off resist 161 to a greaterextent than the photoresist 160, thus producing the sub-etchings 22.

Again in this case, there exists a first monolayer alternative,illustrated in FIG. 20, according to which only the monolayer layer ofphotoresist 160 is produced. The cavities 64′ are in this case boundedby the walls 15 which, on account of the diffraction and reflectionphenomena of the UV radiation already mentioned, present an undercut β,which is positive according to the sign convention indicated in FIG. 20.

The second monolayer alternative also exists, again illustrated with theaid of FIG. 20, according to which only the layer of photoresist 160 isproduced, which is then treated with a surface modifier, for instancetoluene, which renders the upper surface 114 more resistant to thesolvent: following development, in the same photoresist 160, walls 15with sub-etching or with a more accentuated positive undercut β areproduced in this way.

In a step 176, illustrated with the aid of FIG. 21 and similar to thestep 76 already described for the known art, vacuum depositing isperformed, for example of a metal, on the surface 114, where the latterhas remained intact, and on the face 11 where the latter has beenuncovered through the effect of the development described in step 175.Depositing is performed, for instance, by means of a sputtering orelectron beam process, both of which known, the result of which is asubassembly 24, comprising a first deposited layer (52, 54) whichassumes the same geometry as the cavities 64 and effectively constitutesthe outer pad 52 and the inner pads 54, and a further deposited layer116 adhering to the surface 114. No layer, on the other hand, isdeposited on the sub-etchings 22.

Even if the monolayer technology is chosen, no layer is deposited on thewalls 15, thanks to the positive undercut β and the sub-etchings, ifany.

The deposited layers 52, 54 and 116 may be metallic, or made ofnon-metallic materials, such as for example oxides, nitrides, carbidesand the like.

If the deposited layers 52, 54 and 116 are made of metals, these may benon-reactive, for example titanium, gold or platinum, in order toproduce the outer pads 52 or the inner pads 54, or a gold/tin 80/20alloy to produce the solderings. These alloys are usually deposited,according to the known technology, in alternate layers of the componentmetals: the various layers are made with appropriate ratios between thethicknesses to give the alloy—generally eutectic—the right composition,and can assume an overall thickness, for instance, of up to 5 μm.

In a step 177, similar to the step 77 already described for the knownart and illustrated with the aid of FIG. 22, the layer 160 of positivephotoresist and the layer 161 of lift off resist are removed by means ofthe lift off process. The subassembly 24 is plunged into a solvent 36,for example of acetone or, better, Remover PG by Micro-Chem, which,through the edges 25 and the sub-etchings 22, free of the depositedlayer, penetrates through the layers 160 and 161, and dissolves themproceeding in the directions indicated by the arrows 21, eliminatingthem completely and freeing the further deposited layer 116, which iscast aside. The operation is facilitated by a mechanical action such as,for example, an ultrasound washing.

If the monolayer technology is chosen, this step 177 is carried out in asimilar way, since the solvent can penetrate through the walls 15, whichare also free of the deposited layer.

In a step 143, the covering 33 of negative photoresist is removed bymeans of, for example, a known type plasma operation.

At the end of the step 143 the subassembly 24 is finished as shown inFIG. 23, where the die 55, the etching 12, an outer pad 52 and an innerpad 54 can be seen.

1. A method for selectively covering a micro machined surface on a diecomprising an upper face and at least one etched recess located in theupper face, the method comprising: applying a film of negativephotoresist onto the upper face, the film of negative photoresistcovering the at least one etched recess without filling the at least oneetched recess; exposing the film of negative photoresist to ultravioletradiation through a first mask, the first mask having a windowcoextensive with at least the at least one etched recess, therebypolymerizing a region of the film of negative photoresist covering theat least one etched recess; removing a non-polymerized portion of thefilm of negative photoresist from the upper face, thereby leaving acovering of polymerized film over the at least one etched recess;applying a layer of lift off resist over the upper face of the die andover the covering; and applying a layer of positive photoresist over thelayer of lift off resist.
 2. The method according to claim 1, furthercomprising: exposing a portion of the layer of positive photoresist toultraviolet radiation through a window in a second mask to depolymerizethe portion of the layer of positive photoresist; and removing thedepolymerized portion of the layer of positive photoresist, therebyforming at least one cavity.
 3. The method according to claim 2, furthercomprising: applying a first deposited layer on said upper face of saiddie; applying a further deposited layer on an upper surface of the layerof positive photoresist; removing the layer of positive photoresist andthe layer of lift off resist using a solvent that acts through sideedges and sub-etchings of the at least one cavity; casting aside thefurther deposited layer; and removing the film of negative photoresist.4. The method according to claim 1, wherein the first deposited layerand said further deposited layer are metallic.
 5. The method accordingto claim 4, wherein the first deposited layer and said further depositedlayer include at least one layer of gold or of titanium or of platinum.6. The method according to claim 4, wherein the first deposited layerand said further deposited layer include at least one layer of gold/tinalloy.
 7. The method according to claim 1, wherein the further depositedlayer and said first deposited layer comprise non metallic material. 8.The method according to claim 7, wherein the nonmetallic materialscomprise an oxide.
 9. The method according to claim 7, wherein thenonmetallic materials comprise a carbide.
 10. The method according toclaim 7, wherein the nonmetallic materials comprise a nitride.
 11. Amethod for selectively covering a micro machined surface on a diecomprising an upper face and at least one etched recess located in theupper face, the method comprising: applying a film of negativephotoresist onto the upper face, the film of negative photoresistcovering the at least one etched recess without filling the at least oneetched recess; exposing the film of negative photoresist to ultravioletradiation through a first mask, the first mask having a windowcoextensive with at least the at least one etched recess, therebypolymerizing a region of the film of negative photoresist covering theat least one etched recess; removing a non-polymerized portion of thefilm of negative photoresist from the upper face, thereby leaving acovering of polymerized film over the at least one etched recess; andapplying a layer of positive photoresist over the upper face of the dieand over the covering.
 12. The method according to claim 11, furthercomprising: exposing a portion of the layer of positive photoresist toultraviolet radiation through a window in a second mask to depolymerizethe portion of the layer of positive photoresist; and removing thedepolymerized portion of the layer of positive photoresist, therebyforming at least one cavity.
 13. The method according to claim 12,further comprising: applying a first deposited layer on said upper faceof said die; applying a further deposited layer on an upper surface ofthe layer of positive photoresist; removing the layer of positivephotoresist using a solvent that acts through walls of the at least onecavity; casting aside the further deposited layer; and removing the filmof negative photoresist.
 14. The method according to claim 13, whereinthe first deposited layer and said further deposited layer are metallic.15. The method according to claim 14, wherein the first deposited layerand said further deposited layer comprise at least one layer of gold orof titanium or of platinum.
 16. The method according to claim 14,wherein the first deposited layer and said further deposited layercomprise at least one layer of gold/tin alloy.
 17. The method accordingto claim 13, wherein the first deposited layer and said furtherdeposited layer are made of nonmetallic materials.
 18. The methodaccording to claim 17, wherein the nonmetallic materials comprise anoxide.
 19. The method according to claim 17, wherein the nonmetallicmaterials comprise a carbide.
 20. The method according to claim 17,wherein the nonmetallic materials comprise a nitride.